
\begin{tikzpicture}
  \coordinate[label=above:$A$] (A) at (4,3);
  \coordinate[label=below:$B$] (B) at (0,0);
  \coordinate[label=below:$C$] (C) at (4,0);
  \pic[mark angle={green}{3mm}{1}] {right angle=A--C--B};
  \draw (B) -- node[above left] {$c$} (A)
    -- node[right] {$b$} (C)
    -- node[below] {$a$} cycle;
\end{tikzpicture}
